Posttraumatic stress disorder (PTSD) is an extreme and sustained maladaptive response to traumatic events. PTSD is a major cause of mental health morbidity and loss of function in affected individuals. Two symptom clusters - re-experiencing symptoms, which include intrusive memories and thoughts, sometimes triggered by environmental cues, and hyperarousal symptoms, characterized by a tonic state of concern and active monitoring of the environment for threat - are particularly debilitating for maintaining effective occupational and social functioning. Individuals with PTSD are prone to re-experience the trauma in a variety of settings and in response to a variety of cues that often differ from the initial traumatic event. Fear generalization and contextualization paradigms provide models to test hypothesized alterations in functional brain regions and connectivity in PTSD that is biased to interpreting cues in the environment as trauma relevant, thus maintaining re-experiencing symptoms. Hyperarousal symptoms are characterized by constant threat monitoring. A predator stress model, which examines vigilance and active coping responses to imminent threats, is useful to test changes in systems that manage multiple, competing demands created by a threatening environment. We propose to test a neural circuit model that forms the basis of re-experiencing and hyperarousal symptoms of PTSD. The functional components of the network model include a threat-alerting system that consists of the amygdala, extended amygdala/ventral striatum, insula, and ventromedial PFC, and a threat-assessing system that consists of hippocampus, anterior cingulate, dorsal striatum, dorsomedial PFC, precuneus, and ventrolateral PFC. To gain insight into the pathogenesis of the re-experiencing and hyperarousal symptoms of PTSD, we have developed a series of interactive paradigms of fear learning and threat-based arousal for the fMRI setting that contain features of computer gaming and virtual reality to identify the relevant dysfunction in connectivity patterns within the proposed brain network. These tasks will be conducted in Veterans with PTSD returning from post-9/11 operations in the Middle East and combat-exposed controls, and brain network relationships will be correlated with current PTSD symptoms. The insights gained into the pathogenesis of PTSD will lay the groundwork for developing future therapeutic interventions that are targeted to symptoms.